The present invention relates generally to a corrosive and erosive resistant heat exchanger and more particularly to a heat exchanger which utilizes a boundary layer of innocuous gas which is continuously replenished, to protect the surface of the heat exchanger walls from contaminated or corrosive gases.
A heat exchanger, which is capable of recovering heat from a high temperature contaminated or corrosive stream is needed in several industrial and research applications.
Blast furnace stoves, used in the steel industry, are regenerative heat exchangers which are used to heat the blast air supplied to a blast furnace. These stoves include a combustion chamber in which a fuel, typically coke oven gas, is burned with air to provide a heat source. The exhaust stream from the combustion chamber is passed through a matrix of ceramic refractory tiles which absorb heat from the gas stream. The combustor is then shut off and blast air is passed through the tiles, absorbing the stored heat.
Glass melting furnaces use the heat from exhaust gas to preheat the combustion air and sometimes the fuel. Exhaust gas leaving the furnace is passed through ceramic refractory tiles which absorb heat from the waste stream. Heat is then transfered from the tiles to the combustion air or fuel.
Aluminum remelt furnaces use the heat from exhaust gas to preheat the combustion air. Exhaust gas leaving the furnace is passed through stack type or shell and tube type metal heat exchangers where the heat from the exhaust gas is transferred from the gas through a separating metal wall to preheat combustion air.
The exhaust streams of the blast furnace stoves, glass melting furnaces and aluminum remelt furnaces may be highly corrosive and/or erosive. Although ceramic refractory tile materials are somewhat effective against the corrosive and erosive combustion products in either the blast furnace or the glass melting furnace, there are several disadvantages to the use of these materials. Ceramic materials are expensive, particularly when used in large industrial-scale applications. Additionally, ceramic materials cannot be easily formed into large or exotic shapes. If a large diameter duct is needed, it cannot be extruded from a ceramic material. Rather, the duct must be made of a matrix of individual ceramic tiles. Metal ducts or walls can be easily fabricated by an extrusion process. However, as often occurs in metal heat exchangers installed on aluminum remelt furnaces, a metal wall is rapidly corroded by such heat streams.
Magnetohydrodynamic (MHD) technology is another area in which a need exists for a corrosive resistant heat exchanger. The performance requirements of a heat exchanger used in recuperating heat from a MHD waste stream, exceed the requirements of the blast furnace stove, glass furnace or aluminum remelt furnace heat exchangers. First, the temperatures of both the heating gas (approximately 3,000.degree. F.) and the heated gas are substantially higher for the heat exchangers used in MHD applications. Second, the relatively large concentrations of coal ash and seed compounds, such as potassium and cesium, in the MHD gas in conjunction with the high temperature, make the waste stream much more corrosive and erosive than the combustion products in a blast furnace, a glass melting furnace or aluminum remelt furnace. These conditions may have deleterious effects even on ceramic materials.
U.S. Pat. No. 4,365,543, issued to Maurice R. Baker, discloses an arrangement which may be useful in protecting the walls of a duct from a corrosive stream. In the Baker arrangement, a corrosive waste gas is maintained separate from the structural wall of a chimney or duct by a continuous jacket of air which moves along with the waste gas. The air forming the protective jacket is introduced into the duct through inlets around the perifery of the duct. Rings, supported by ribs, are provided at spaced intervals near the inner surface of the duct. The rings prevent the air from drifting radially inward, thereby forming a protective air layer between the duct wall and the waste stream.
The arrangement disclosed by the Baker patent, however, may not be suitable for the industrial or MHD applications discussed above. The rings in the Baker arrangement are themselves in direct contact with the waste stream. A hot corrosive or erosive waste stream would have the same effect on the ring material as on a duct wall. Additionally, the ring arrangement may not be sufficient to prevent a turbulent waste stream from mixing with the protective air layer. Thus, the ring arrangement may not be effective in preventing corrosive waste stream particles from impinging on the duct walls, if the waste stream flow is turbulent. Further, frictional drag between the air and the wall will cause the protective air layer to decay after a given length of duct. The length of the heat exchanger duct is therefore limited in this arrangement.
Therefore, in view of the above, it is an object of the present invention to provide a corrosive and erosive resistant heat exchanger.
It is another object of the present invention to provide a heat exchanger capable of recovering heat from a hot corrosive waste gas stream.
It is still another object of the present invention to provide a heat exchanger in which a hot corrosive waste stream is prevented from contacting the heat exchanger walls.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.